R6(a): de-stage code, docs, READMEs; split stage6_test

Mechanical, behaviour-preserving removal of Stage N / TODO-N / phase (RN)
references from comments, doc-comments, service READMEs, the current-state docs
(ARCHITECTURE, FUNCTIONAL+_ru, TESTING, UI_DESIGN), config-file comments, and the
.fbs/.proto schema comments. PLAN.md / PRERELEASE.md / CLAUDE.md keep the stage
history.

- Rename the only stage-named identifiers: registerStage8 -> registerSocialOps,
  registerStage11 -> registerLinkOps (gateway transcode).
- Split stage6_test.go: TestEmailLoginFlow -> email_test.go,
  TestGuestAutoMatchLeavesNoStats (+ provisionGuest) -> account_test.go.
- Regenerated proto bindings (push.pb.go, telegram_grpc.pb.go) from the de-staged
  .proto comments; FB Go/TS bindings unchanged (flatc strips schema comments).

go build/vet/gofmt clean across modules; integration typecheck and pnpm check green.

backend/internal/notify/notify.go

@@ -28,7 +28,7 @@ const (
 	// (incoming friend requests, invitations) that drives the lobby badge.
 	KindNotification = "notify"
 	// KindGameOver announces a finished game to each seated player, driving the
-	// out-of-app "game over" push (Stage 17).
+	// out-of-app "game over" push.
 	KindGameOver = "game_over"
 )
 
@@ -52,7 +52,7 @@ type Intent struct {
 	Kind    string
 	Payload []byte
 	EventID string
-	// Language routes an out-of-app push to a specific per-language bot (Stage 17): for a
+	// Language routes an out-of-app push to a specific per-language bot: for a
 	// game event it is the game's language ("en"/"ru"), so the notification comes from the
 	// game's bot rather than the recipient's last-login bot. Empty falls back to the
 	// recipient's service language at the gateway.